This invention relates to inkjet printers and, more particularly, to an improved large-format digital color inkjet printer including a group of sensors and subassemblies that cooperate to produce high quality graphic images using a plurality of different colors of ink and different types of print media at speeds several times faster than similar conventional inkjet printers. In addition, the use of cooperating elements permit the manufacture of complex, large-format digital color inkjet print engines that are less expensive to fabricated, operated, and serviced. The present invention finds use in large-format digital color printing and imaging, where successful repeatable printing requires precise placement of droplets of ink, toner or other marking material on a print medium such as paper, vinyl, film, or similar substrate.
Inkjet printers are well known. Large-format inkjet printers generally move a scanning carriage containing one or more print-head in a transverse or horizontal direction across a print medium, while incrementally advancingxe2x80x94or xe2x80x9csteppingxe2x80x9dxe2x80x94a print medium in a lengthwise or vertical direction in-between successive printing passes, or scans, of a reciprocating carriage. Inkjet printing involves placing large quantities of tiny ink droplets formed by one or more ink-emitting (or xe2x80x9cjettingxe2x80x9d) nozzles onto predetermined locations on a print medium or substrate. The ink droplets solidify or dry on the print medium forming small dots of color. A quantity of these small colored dots when viewed at a nominal distance will be perceived as a continuous-tone visual image. To increase the rate of print production, a print-head typically employs numerous jetting nozzles per color of ink ganged together in a suitable arrangement to create a band or xe2x80x9cswathxe2x80x9d of printed area that is much wider than otherwise would be obtainable from a single jetting nozzle. Usually, several linear arrays of jetting nozzles are disposed in a print-head in an orientation parallel to the direction of media travel (X-axis) and perpendicular to the direction of carriage travel (Y-axis). Both text and graphic images may be printed with inkjet printing.
The printed image from an inkjet printer is made up of a grid-like pattern of potential dot locations, called picture elements or xe2x80x9cpixelsxe2x80x9d. A pixel generally refers to a coverage area that is defined by the incremental advance accuracy of the media (positioning resolution) of the media drive system along the X-axis, and the maximum number of colored dots the print-head can produce (marking resolution) along the Y-axis. Pixel density is often referred to as print resolution; while pixel density is often the same for both travel axes, this need not be the case for every inkjet printer. Print resolution is often conceived of as a performance measure of an inkjet printer. The print resolution of an inkjet print engine tends to vary as needed for the particular imaging application; hence, the print resolution necessary for printing a billboard, such as are commonly viewed from hundreds of feet away, may be on the order of 6-12 pixels per inch. For many smaller-format documents commonly viewed from 1-6 feet away, the inkjet printing industry has produced printer with a print resolution of between 200 and 2600 pixels per inch (40,000-6,760,00 pixels per square inch) and a maximum media width of 18 inches. While the aforementioned upper range of print resolution may be acceptable for smaller-format documents, such as photo-reproduction or color-matching images, which have an optimum viewing distance of less than 1-8 feet, the use of higher print resolution in large-format devices becomes problematic for a number of reasons.
One reason, which tends to obviate any others, is that employing higher print resolution in larger-format images tends to be counter-productive due to the large amount of image data that must be processed. In digital printing, example, for every doubling of print resolution there is a concomitant quadrupling of the number of required pixels for the same printed area (e.g., 100xc3x97100 dpi=10,000 pixels per sq. in.; 200xc3x97200 dpi=40,000 pixels per sq. in., etc.). Each pixel requires at least one memory location to represent it: hence, each time print resolution doubles the number of memory locations required to render the same size image quadruples. Data inflation not only affects how much expensive on-line memory is needed to render an image, but also influences various other aspects of printer design and manufacture. For example, higher print resolution requires a fast I/O system to handle the large amount of image data that must be transferred from a rendering device to the print heads, as well as a fast processor and optimized software to quickly render a large image for printing. Higher print resolution also requires large off-line storage devices such as hard-disks and CD-ROM drives to store a rendered image, as well as large data buffers to stage the rendered image data to the print heads. Each of these outcomes tends to increase the cost of manufacture for large-format, graphics-quality inkjet printers.
The problem of data inflation is further exacerbated by the use of process color printing. Inkjet printers generally use one or more of several different types of ink and potential combinations of colors of ink. Color inkjet printers of the prior art typically use the four subtractive primary colors: cyan, magenta, yellow and black (xe2x80x9cCYMKxe2x80x9d). Color blending of these four ink colors is achieved through two mechanisms. First, the inkjet printer may deposit multiple colors of ink on the same pixel location. Upon combining one or more of the CMYK ink colors at a given pixel location, a particular color combination is formed, either in a dot-on-dot or a dot-next-to-dot pattern. The particular color combination produced by depositing multiple ink colors at a particular pixel location may be affected by the order of printing the various colors, as well as the homogeneity (or lack thereof) of ink mixing. Second, when viewed at a distance, the eye will blend colors from adjacent pixel locations. Thus, for instance, a number of exclusively magenta and yellow colored dots may be laid down in an area of a printed image, with no pixel location receiving two colors of ink. Rather than perceiving individual magenta and yellow dots, the eye will blend the adjacent dots to perceive an orange color. In practice, ink blending at particular pixel locations and perception blending across pixel locations are used to create various colors and shades in a printed image. Usually, a substantial number of the pixel locations in a printed image will be left blank, allowing the perceived visual image to have the correct shades or tones (lightness/darkness values) across the image. Through both forms of color blending, inkjet printers using only four colors of ink can visually reproduce continuous-tone, graphics-quality color images.
However, for every individual color used to render an image, a separate pixel gridxe2x80x94or color planexe2x80x94must be rendered and staged in either on-line or off-line memory, or both, for transmission to the print-head. Consequently, the amount of memory needed to render and store image data correspondingly increases for each additional color of ink used to print an image. For example, a 36xc3x9742-inch image will comprise 1512 square inches of printed area. At a print resolution of 600 dpi, this image area constitutes a pixel grid having 544,320,000 discreet grid locations, or pixels (i.e., 1512xc3x97600xc3x97600=544,320,000). For each color used to print an image, a separate color plane must be rendered that controls whether a drop of ink of a particular color willxe2x80x94or will notxe2x80x94be deposited at each specific pixel grid location. Thus, to render a CMYK image at 600 DPI requires four separate color planes and generates 2,177,280,000 (c.f, 544,320,000xc3x974=2,177,280,000) bits of total image data. As print resolution increases, or as the number of process colors used to print an image increases, or both, the amount of data needed to represent the image increases accordingly.
Increasing the print resolution for large-format inkjet printers is contra-indicated for other reasons as well. As print resolution increases, either the jetting nozzles in a print-head must fire faster to place the ink drops in a correspondingly smaller and increased number of pixel locations, or the speed of the scanning carriage must be slowed. For example, when print resolution doubles the firing rate of the inkjet print-head also must double, or the scanning speed of the carriage must be halved, to ensure that the print-head has adequate time to precisely deposit a drop of the proper color ink at every predetermined pixel location. Obviously, reducing the scanning speed of the carriage will result in a comparable increase in the amount of time it takes to print an image; put another way, the printer""s output speed or rate-of-production will decrease accordingly.
Since users of large-format digital color printers will find any reduction in output speed unacceptable for economic reasons, manufacturers of inkjet print heads tend to increase the firing rate of a print-head commensurately with an increase in print resolution. In addition, increases in print resolution and print-head firing rate usually are accompanied by a decrease in the volume of ink being discharged from the jetting nozzles, since the amount of area in a pixel location is proportionately smaller. For example, a hypothetical 600 DPI print-head with 240 jetting nozzles and a firing rate of 5.6 kHz might discharge an ink drop with a nominal drop volume of 32 nanograms. Doubling the print resolution to 1200 DPI might result in a print-head with 480 jetting nozzles and a firing rate of 11.2 kHz, accompanied by an approximate halving of the nominal drop volume to 18 nanograms.
Clearly, an increase in print resolution dictates a number of requisite characteristics in the design of large-format digital inkjet printers. For example, inks must be formulated that complement both the operating dynamics of the print-head jetting nozzles, as well as the absorptive characteristics of one or more print media. The positioning system of the scanning carriage must be sufficiently accurate and dimensionally stable to precisely deposit a smaller-volume, higher-velocity ink drop in a correspondingly smaller pixel location. The media drive system must be sufficiently exact to more accurately advance a print medium a precise distance between successive reciprocation of the scanning carriage. The printer control electronics must be sufficiently advanced to transmit image data to the print heads as needed to maintain print production without pauses, as well as sophisticated enough to control complex printing operation through the use of sensors and monitoring devices.
The difficulty in effecting any of these requisite characteristics is compounded by challenges inherent in the design of large-format inkjet printers over their small-format counterparts. The deposition of ink droplets in a pixel location must be very carefully controlled over a much larger marking area to create a high-quality image. A related challenge involves the means used to mount and orient the print heads within the carriage to precisely position them relative to each other and to ensure accurate placement of ink drops as the carriage moves across the printed image. Also, large-format printers require a consistent, accurate position-feedback method across a broad expanse of carriage travel to determine when the jetting nozzles should be fired based on the location of the print-head with respect to the printed image. While is it known that accurate positioning of the print-head is key to precise placement of ink drops in a pixel location, this becomes more difficult as the scanning distance (i.e., the width of the print medium) increases and more print heads are used (i.e., the width of the carriage increases). Since the scanning carriage is a reciprocating device, the carriage support structure, or rail, must accommodate a travel margin at each end sufficient to allow the carriage to completely pass over either edge of the print medium. This margin generally is used for xe2x80x9cturn-aroundxe2x80x9d space wherein all print heads may pass over the full width of a print medium at the nominal printing velocity, the carriage may then come to a halt, and turn around to begin reciprocal travel. In addition, the rail must adequately support the carriage not only across the entire width of the print medium, but also to accommodate any cleaning, maintenance, or other auxiliary functions that may be required to service the print heads. Therefore, it is common in the art to provide a service zone, commonly called a xe2x80x9cmaintenance stationxe2x80x9d, away from the print medium where the printer may station the carriage to park the print heads when not in use, or to perform other auxiliary service functions. Service functions may include cleaning and capping of the print heads, replacing print heads and related components, cleaning and adjusting on-carriage sensors, adjusting the height of the print heads relative to a given print medium, adjusting the print-head axial orientations relative to one other, and performing various calibrations of print heads, among others. Therefore, to incorporate a maintenance station or service zone, the rail must support the carriage over a distance greater than the width of the print medium by at least the width of the scanning carriage itself.
The design of a scanning carriage and supporting rail for use in a large-format inkjet printer encounters additional challenges in the form of limitations and instabilities due simply to the length and mass of structural members, the travel distance, and difficulties related to precisely controlling various tolerances in manufacture. Inkjet printers often have problems in aligning and orienting the inkjets that are not easily correctable through mechanical manipulation of the print-head position. These problems are aggravated as the number of print heads increases, as the relative spacing between print heads changes during replacement of the print heads, and as the ink delivery and mechanical placement of print heads becomes more complicated. A known phenomenon called xe2x80x9ctolerance stackingxe2x80x9d contributes a significant error component in an assembly process wherein at least two precision machining events occur at different times. In the manufacture of a scanning carriage for a large-format inkjet printer, such tolerance stacking may occur at a number of discrete points in the fabrication of related subassemblies. Consequently, machining tolerances specified for various subassemblies, no matter how rigorous, may be either additive or reductive in contributing significant positioning error when arriving at an exact location and orientation of the print heads relative to one another and to the print medium.
Other challenges lie in requisite design elements of the scanning carriage. For example, for large-format printing, a carriage typically must support and precisely orient at least onexe2x80x94and perhaps as many as twelvexe2x80x94inkjet print heads, a portion of the circuitry for controlling multiple print heads, any on-carriage sensors, and apparatus for driving the carriage along the rail in a precisely controlled manner. Moreover, the carriage may support one end of a guide-way or track that contains multiple electrical cables supplying power and signals to the carriage, as well as ink supply tubes supplying ink from off-carriage reservoirs to the print heads. This track applies certain inertial, frictional and/or oscillation forces to carriage motion beyond those inherent in driving the carriage itself. Translational forces may result in vibration problems if the carriage sustains unrestrained movement, causing the print heads to articulate slightly about axes both parallel and perpendicular to the rail, causing the print-head placement with regard to the print medium to be inaccurate. Also, the carriage may shake slightly from side-to-side on the rail in the direction of travel, perhaps due to undamped oscillations communicated from a drive belt and idler apparatus typically used to drive a scanning carriage, as is known in the art. Additionally, carriage position is typically determined by optically sensing indicia from an encoder strip. The encoder indicia are intended to reside at precise intervals along the length of the encoder strip. The optical-sensor produces a signal as the scanning carriage changes location along its travel and the print heads are fired based on the position-feedback data reported. While encoder strips thus may provide means to determine when print heads should be fired, various fabrication errors can occur which prevent the encoder strip indicia from representing exact intervals corresponding to a precise position for firing an inkjet nozzle over a pixel location.
Yet further challenges lie in the means employed for incrementally transporting a print medium during successive passes of the scanning carriage during printing operation. Ideally, inkjet printing is accomplished using vertically aligned jetting nozzles (i.e., parallel to the X-axis), with each nozzle positioned a pixel-interval below a preceding nozzle. In fact, inkjet print heads typically employ numerous jetting nozzles per color in this configuration to facilitate printing in a band of printed area per pass of the scanning carriage, as previously described. Unfortunately, this configuration often predicates what is known in the art as xe2x80x9cbandingxe2x80x9d, a pernicious printing irregularity or artifact that is common to inkjet print engines in general, and large-format printers in particular.
One type of banding artifact occurs if the media drive system is not extremely accurate, such that the print medium is advanced slightly more or slightly less than the width of the print-head xe2x80x9cswathxe2x80x9d or printed band (i.e., the vertical extent of the line of jetting nozzles). If the print medium is advanced slightly too far, a perceptible blank area will occur in the printed pattern at the end of each advance, between two successive print swaths. On the other hand, if the print medium advance is too short, a perceptible darker area will occur in the color pattern at the beginning of each advance, where adjoining print swaths overlap slightly. Banding that occurs due to media advance inaccuracies often is related to variations in the type of media used. Different media types have different handling characteristics and will react to exposure to heat, ink, and tensioning forces in a variety of different ways. Variations in the thickness and stiffness of two different print media often will result in different feed rates through the printer. For example, as known in the art, a print medium typically is advanced through the printer by a drive motor that rotates one or more drive wheels in contact with the medium at a pinch point, or nip. The printing medium typically is forced against the drive wheel by a pinch-roller, or other suitable mechanism. The pinch-roller typically is formed fromxe2x80x94or has deposited on its outer surfacexe2x80x94a hard rubber material, while the drive wheel typically is formed fromxe2x80x94or has deposited on its outer surfacexe2x80x94a rough material, such as grit, suitable for gripping and advancing the medium. Depending upon the thickness, stiffness, frictional coefficient of the paper, and the force exerted by the pinch-roller on the print medium, the rubber surface of the drive wheel is deflected by varying amounts. This deflection phenomenon results in a slight increase in throughput speed of the print medium, due to differential compression forces applied to the print medium and the drive wheel from the pinch-roller. Consequently, the distance that the drive wheel advances any given print medium for any given number of rotations of the drive wheel may vary, resulting in different rates of advance for two different print media. Another type of banding artifact may be caused by differences in the tension of the print medium. In particular, the accuracy of print media advance is influenced by the differential tension across the nip-point. That is, the difference between the tension in the print medium on the supply side and the take-up side of the nip-point affects the rate at which the print medium is advanced by the drive rollers. If the differential tension across the nip-point continuously changes during printing operation, the rate of advancement of the printing medium will also continuously change.
Various methods have been attempted to compensate for the above-cited banding problems. One such method is referred to as xe2x80x9cmulti-passxe2x80x9d printing. In multi-pass printing, the print medium is advanced at a fractional increment of the print-head footprint, or print swath, such that two or more jets of the same color pass over any given pixel row in sequential passes of the scanning carriage. The first jet prints only a portion of the colored dots on that particular pixel row, with remaining dots on the pixel row printed on subsequent passes. Multi-pass printing tends to mask banding artifacts that result from small media advance inaccuracies such that they are not easy to perceive in the printed image. However, multi-pass printing also significantly increases the time it takes to print an image, resulting in a decrease in output speed.
In actual practice of the art such considerations are fundamental. Any of the foregoing inherent design challenges in the manufacture of large-format inkjet printers may produce printing errors and artifacts, including banding, which tend to be exacerbated as the speed of printing, the size of output, the number of print heads, or the print resolution are increased. It follows, then, that a significant increase in print resolution will tend to amplify or magnify even small inaccuracies, variances, or flaws in critical components and assemblies, which will evidence directly as artifacts in the printed image. In effect, the printed image itself becomes a measure of printer performance and proves the inadequacy of existing solutions to meet the new threshold of performance.
Moreover, an increase in print resolution tends to exclude design solutions embodied in the prior art for the same reasons. High speed, large-format inkjet printing requires a high degree of accuracy to generate graphics-quality images. As a general rule, the cost of manufacture increases as the design tolerances of mechanical systems increase (i.e., demand for accuracy or precision in the fabrication of components and assemblies). Large-format inkjet printers are no exception. In sum, various systemic limitations of designxe2x80x94both individually and in concertxe2x80x94may impede the successful translation of increased print resolution to image quality in a large-format print engine. Hence, the limit of print resolution is not simply what can be achieved in fabrication of a print-head, but instead may also include the resolution needed to render acceptable print quality for the image desired at a sustainable cost of manufacture.
Print resolution in the prior art generally has been understood as beneficial in rendering fine detail in a printed image. However, in printing large-format images, fine detail tends to be enlarged proportionately along with gross detail. Since the human eye is incapable of distinguishing between higher print resolutions (e.g. 600 dpi and above) at distances of more than a few feet (e.g., 10 feet or more), there is little benefit in using a higher print resolution merely to improve the quality of fine detail. Consequently, there must be other relevant purposes in increasing print resolution, such as improving the visual quality of the printed image or increasing the print speed of a print engine.
One means to improve image quality using increased print resolution lies in replicating the tonal variations of a continuous-tone source image. It is well known in the art that tonal variations of a source image may be replicated by approximating the tonal values with corresponding shades or densitiesxe2x80x94lightness/darkness valuesxe2x80x94in the printed image. It is less well known that tonal variations in a source image also may be replicated, or enhanced, using relative variations in hue in the printed image. This method of replicating a continuous-tone image results in higher-quality output, since the printed image is more pleasing to the eye. However, this methodology generally requires an extended set of process ink colors, beyond the standard set of CMYK colors. One such extended set of process colors current in the art includes light and medium hues of cyan and magenta inks, effecting an eight-color set (i.e., cyan, magenta, yellow, black, light-cyan, medium-cyan, light-magenta, medium-magenta). Another extended set of process colors current in the art includes red, orange, green and blue inks, effecting a twelve-color set (i.e., cyan, magenta, yellow, black, light-cyan, medium-cyan, light-magenta, medium-magenta, red, orange, green, blue). Either of these extended ink sets will allow replicating tonal variations of a continuous-tone source image using hue values, as opposed to (or in addition to) the use of density values, in a printed image. A related benefit of using an extended set of process ink colors is to extend the color gamut that can be produced by the inkjet printer to more closely approximate that of the source image.
Increased print resolution also enables an increased number of colored dots to be applied to a pixel location in creating a given color or hue, thereby allowing a wider range of hues to be used in replicating a range of tonal values. Obviously, to exploit an increase in print resolution to produce high-quality graphic images in this way requires a scanning carriage that can accommodate an entire set of extended process ink colors (i.e., multiple print heads) simultaneously. Additionally, the printer control system for an inkjet printer of this kind must be capable handling the large amounts of image data that must be transmitted to the print heads, since each pixel location will be represented in as many as 12 different color planes. Further, it is desirable that the printer control system is able to sense or monitor hue values and effect means to automatically effect color-metric adjustments to ensure those tonal values of the source image are consistently reproduced.
It is known that different print media will produce variations in color hues when used with a given set of process inks. Color variations between different media may be due to differences in the physical and chemical interactions between the inks and the print medium, such as the composition of substrates or coatings, the porosity of the medium, or environmental conditions such as the relative humidity. While variations in color may be considered negligible in some applications, in the production of large-format graphics quality images even minor color variations are unacceptable. For this reason, it is desirable to precisely control many of the print engine""s operating parameters, as well as to fully characterize the print medium and the ink, to enable accurate reproduction of a desired image. Because differences in ink colors are easily detectable and provide ready demarcation points relative to the individual print heads, it is common to calibrate the print-head position using test patterns printed in the corresponding ink colors. For example, a test pattern may be printed from each of the print heads and compared to determine the degree to which each is positioned correctly relative to one another. The printer control system may then compensate for unaligned jets or inaccuracies in print-head position by adjusting the timing signals that control when the jetting nozzles are fired, based on the print-head location with respect to the pixel location. Similar tests may be used to confirm that a threshold of functioning nozzles is met for each print-head and that combinations of ink colors applied to a given print medium accurately replicate a desired color. Horizontal travel and vertical advance accuracy can be tested and calibrated in a similar ways.
Because calibrating a printer""s actual operating parameters is an important part of producing high-quality images, operators spend significant time performing the various calibration routines. In response, the inkjet printing industry continually seeks new and better ways to readily determine what calibration adjustments are needed. It is therefore desirable that any of these functions could be performed automatically by the printer control system, without requiring intervention by an operator.
A second relevant factor for increased print resolution is increased print speed. Here again, the use of a scanning carriage that employs multiple print heads finds application. Multiple print-head locations in a scanning carriage will permit the use of multiple ink sets of the same color in printing an image. For example, a scanning carriage with twelve print-head locations could accommodate three sets of CMYK inks, or two sets of hexachrome inks (e.g., CMYKOG). Accordingly, in a multi-pass inkjet printer the same area can be printed using fewer passes of the carriage, since more jetting nozzles of the same color ink will pass over a given pixel location. However, since more jetting nozzles are being used at an increased firing rate, ink will be applied very quickly and the jetting nozzles will age and fail at a faster rate. These effects are attributable to rapid fatiguing of the mechanical and electrical elements of the jetting nozzles, which causes drop volume and placement accuracy to degrade over time.
Aging and failure of the jetting nozzles manifests in the printed image as colored dots that are deposited out-of-position (misfiring jets), or are simply missing altogether, and in the print irregularities or artifacts that thereby result. It is known that multi-pass printers are able to disguise the failure of a nominal percentage of jetting nozzles, since two or more jets pass over any given pixel location. To extend the useful life of a print-head, it is also known that operating jets may be substituted for failed ones in a multi-pass print mode. Further, multi-pass printing allows an operator to select a print control parameter of an inkjet printer whereby the number of scans or passes of the carriage may be increased by some integer as the percentage of jetting nozzle failures increases. While multi-pass printing is an efficient means of counter-acting degradation of image quality due to print-head aging, the improvement in print quality is bought at the price of print speed. A related problem is that nozzle failure tends to increase very rapidly after a certain point is reached in print-head service life. Since only a fraction of the total number of jetting nozzles need fail before the print-head is rendered unserviceable, this phenomenon makes it more difficult to run an inkjet printer in unattended operation, such as overnight, since a failed print-head can result in unusable printed output. Therefore, in applying print heads with increased print resolution to accelerate print speed, it is desirable that print heads may be replaced quickly and easily. It is also desirable that the printer control system be able to sense when a certain number of nozzle failures has occurred and automatically effect a remediating action in response.
It is known in the art that a plurality of disposable inkjet print cartridges may be used in a scanning carriage. Disposable print cartridges typically contain a print-head and a supply of ink contained within the cartridge and are not designed to be refilled; that is, when the internal ink supply is exhausted, the print cartridge generally is thrown away and a replacement cartridge installed in its place. This waste of resources is unacceptable in the design of large-format inkjet printers, since frequent replacement of the print cartridges will result in high operating costs and since a print cartridge generally has a greater useful life well beyond that needed to exhaust its internal ink supply. Because increasing the ink supply of the print cartridge would increase the total weight of the carriage, it is known to use a set of off-carriage ink reservoirs to provide a continuous supply of replenishing ink to the inkjet print cartridges resident in the carriage. A tube hermetically connected extends from each off-carriage ink reservoir to a print cartridge of the same ink color residing in the carriage, thereby establishing fluid communication between the two components. In the art, the ink supply system and inkjet print cartridge generally are removed and replaced together. In application of a disposable inkjet print cartridge with increased resolution to an inkjet printer with higher print speed, it is desirable that an ink supply system be reliable enough to sustain uninterrupted delivery of replenishing ink on-demand, yet flexible enough to ensure rapid replacement of print heads and related components.
While the type and number of inkjet print cartridges available for use in the design of large-format inkjet printers has increased, so has the complexity of controlling interactions among the various inks, print cartridges, print media, and related elements. As print-head resolution increases and the service life of the jetting nozzles improves, limitations have arisen in the efficacy of prior art mechanisms and apparatus for fully exploiting the benefits that may be derived therefrom. In general, prior art inkjet printers are not designed for rapid and efficient in-field replacement of critical subassemblies and components requiring limited operator intervention and a minimum loss of production. In fact, due to the obvious competing design objectives of mechanical positioning accuracy and field replacement convenience, little has been accomplished in this regard. Likewise, little has been accomplished in automating the many required service routines for effective use of prior art printers, which would result in reduced print engine down-time, fewer service interventions, and more efficient repairsxe2x80x94thereby reducing the overall cost of ownership of print engines of this kind. At the same time, continuing demand for reduced cost of ownership and ease of serviceability continues to inspire innovation in the art. Thus, a continuing need exists for a low-cost, large-format digital color inkjet printer that satisfies most or all of the deficiencies now current in the art, while at the same time providing a technically advanced means for producing high quality, large-format digital color images.
This invention relates to inkjet printers, and, more particularly, to a co-operating group of sensors and subassemblies that address numerous problems of prior art printers discovered in the design of large-format, graphics quality, digital color inkjet printers that use disposable print cartridges mounted in a scanning carriage. The inkjet printer of the present invention features a group of co-operating sensors, controls and subassemblies that exploit recent advances in the art as embodied in a preferred disposable inkjet print cartridge. The preferred inkjet print cartridge more than doubles the number of ink-emitting jet nozzles, trebles the jetting nozzle firing rate, increases the print-head longevity by several times, and doubles the marking footprint of prior art disposable inkjet print cartridges.
The use of co-operating sensors and subassemblies in a print engine permits the manufacture of an advanced, large-format inkjet print engine that more fully exploits the benefits and advantages inherent in the preferred high-resolution print cartridge, but still may be inexpensively fabricated, operated and serviced. The problems inherent in implementing an inkjet print cartridge having a higher print resolution and a faster firing rate are addressed by dealing with error components across a range of mechanical systems, as opposed to isolating a preponderance of error components within a few expensive machine elements. One advantage of this approach lies in the economic benefit to the ultimate end user of large-format inkjet printers. For example, one difficulty in producing high quality printed images in large-format printers using higher resolution inkjet print-heads, or faster print-heads, lies in precisely positioning the ink-emitting nozzles relative to each other and to the print medium. It lies within the realm of good engineering practice to design an apparatusxe2x80x94such as a penholderxe2x80x94which adheres to very strict design tolerances in fabrication thus ensuring that the cartridge print-head position approaches some ideal. However, this is likely an expensive solution and one not likely to be adaptable to technical advances.
A better approach lies in using both design tolerances and control system software to accomplish an effective countermeasure that is more adaptable and less costly. The co-operating subassemblies of the present invention include an auto-adjusting head-height apparatus, a modular off-carriage ink supply, an auto-adjusting service station, a reliable and efficient media-handling system, an accurate media-drive system, and a sophisticated printing control system that individually and cumulatively provide improvements over the prior art. Together with various sensors and controls, these co-operating subassemblies enable sophisticated monitoring and precise control of various critical printing parameters. These elements cooperate to produce an inkjet print engine that can print high quality, large-format graphic images using up to 12 different colors of ink and different types of print media up to xc2xc-inch thick at speeds several times faster than similar print engines now current in the art. Moreover, the present invention incorporates a number of novel design features that augment its usefulness and operating simplicity, such as a rotating penholder that facilitates service of the preferred inkjet print cartridges and a compliant service station module that automatically accommodates variations in carriage head-height position without requiring operator intervention. Accordingly, inkjet printers of the present invention may be produced in large numbers, at reduced cost of manufacture, making ownership less expensive and operation easier to perform.
The present invention finds use in the large-format digital color inkjet printing industry, where successful repeatable replication of source images requires precise placement of droplets of ink or other marking material on a print medium such as paper, vinyl, film, or coated substrate. In one embodiment, the inkjet printer of the present invention is an improved, large-format, multipass, digital color inkjet printer capable of handling media widths up to 72 inches wide at a minimum print resolution of 600 dots-per-inch. In an alternate embodiment, the inkjet printer of the present invention has a resolution of 1200 dots-per-inch.
The scanning carriage assembly is equipped with an automatic head-height adjusting apparatus that supports the preferred inkjet print cartridges in close proximity to any of several different types of print media up to xc2xc-inch thick including paper, vinyl and fabric. The head-height adjusting apparatus operatively couples a moveable penholder to a sliding trolley plate via two axial drive-screws driven by servomotors under control of the printer control system. The trolley plate and attached penholder is supported on a rail member via a plurality of trolley wheels that engage the rail along a plurality of parallel linear datum structures that permit travel along the X-axis for the scanning carriage to fully traverse a print medium. The servomotors drive the axial drive-screws that operatively engage structural members fixed to the penholder to precisely control the vertical position thereof along the Z-axis in response to position signals transmitted from the printer control system. The printer control system calculates optimum head-height position based on an analog control signal received from a media thickness sensor mounted on the rail that indicates the thickness of media currently loaded in the print engine. An on-carriage image sensor provides means to automatically measure and compensate for print-head position inaccuracy, inkjet nozzle failure, and chromatic variation in the printed image quality.
The scanning carriage preferably includes a penholder that supports as many as twelve preferred inkjet print cartridges at one time to maximize image quality and output speed. The preferred penholder provides mounting locations to precisely position the preferred inkjet print cartridges relative to each other. The preferred penholder also rotates through a travel range of about 85 degrees to allow an operator ready access to the inkjet print cartridges for cleaning, maintenance, or service operations without having to remove and reinstall the print cartridge, or recalibrate the print head position. Structural features formed at each end of the penholder provide means to avoid head-strike conditions due to differences in media thickness, low ambient humidity (causing media curl), and similar conditions. It should be noted that twelve cartridges presents a limitation only insofar as this value currently represents the largest number of process ink colors that can be efficaciously combined to produce large-format color prints. If the technology of process color blending were sufficiently advanced to merit the use of additional print cartridges beyond twelve, the preferred print engine of the present invention would be able to accommodate those colors as well.
The off-carriage ink delivery system includes a modular system including a set of independently replaceable components for each different color of ink. The modular set of components includes an on-carriage inkjet print cartridge, an off-carriage ink reservoir, and an interconnecting tube with fluid connectors and check valves, each of which may be installed and removed, either together or independently, by an operator of the inkjet print engine instant. The preferred inkjet print cartridge includes a print-head, an internal cavity containing a first quantity of ink which directly provisions the print-head and the ink-emitting jet nozzles thereof, and an inlet port for receiving a replenishing supply of ink from the off-carriage ink reservoir via the interconnecting ink supply tube. The off-carriage ink reservoir includes sufficient ink to completely replenish the first quantity of ink within the inkjet print cartridge multiple times, thereby maximizing the service life of the print-head and optimizing the delivery of ink to the print cartridge to assist uninterrupted printing of large-format graphic images. The ink supply tube is equipped with quick-release, fluid connectors with check valves at each end permitting rapid interconnection of the inkjet print cartridge and ink reservoir. This modular system design enables an operator to quickly replace individual components as needed due to aging or failure, as well as an entire set of components when different printing needs arise, such as, for example, when switching between indoor dye-based inks and outdoor pigment-based inks.
The modular service station is equipped with automatic means of adjustment, being suspended on spring-loaded cam rollers that automatically adjust the elevation of the service module to compensate for variations in carriage head-height position. The cam rollers also articulate the service module through a complex travel path defined by structural datum integral to the platen and station it through a series of four distinct operating locations. The scanning carriage visits the service station at intervals under direction of the printer control electronics and drives the service station module along its travel path to its operating locations. The service module performs wiping and capping service routines while maintaining optimum spacing between the inkjet print cartridge disposed in the penholder assembly and the wiper blades and capping boots located on the module. The modular construction embodies a reliable but inexpensive field-replaceable unit that enables an operator to quickly replace it as needed due to aging or failure.
The print medium handling system functions to transport the print medium through the printer. The printing medium handling system includes tensioning means to maintain a constant back-tension in the media web from the supply side of the nip-point as the print medium is incrementally advanced past the print heads. The media handling system includes a sensor that gauges the thickness of a media currently loaded in the printer and communicates thickness data to the printer control system. The printer control system uses the thickness data to control the incremental advance of the print medium in-between successive passes of the scanning carriage. It also uses the thickness data to perform automatic adjustment of the carriage head-height position. The media handling system increases the number of nip-points (e.g., roller pairs) across the media web over prior art printers as one means to mitigate positioning inaccuracy due to deflection phenomenon. A plurality of hard aluminum drive wheels are coated at the tread surface with tungsten-carbide alloy applied using a high granularity heat-sputtering process to provide a xe2x80x9cgrittedxe2x80x9d tread surface, which further reduces deflection phenomenon. A closed-loop servomotor and encoder with quadrate-readout drives a media take-up spool and monitors the take-up roll diameter to control tension, detect faults, and signal failure.
The media drive system accurately transports and precisely positions a print medium within a print zone and optimizes the response time of a media drive train to effect accurate media advance within a limited operational window. A servomotor and reduction gearing generate the low-end torque required to overcome inertial and frictional forces presented by the media handling system that resist a rapid response time. The media drive system accommodates the longer incremental media advance predicated by the greater number of jetting nozzles disposed in the preferred higher-resolution inkjet print cartridge. At the same time, the system optimizes media advance accuracy through the use of a quadrate-readout encoder providing a granularity of about 0.00002-inch. Periodic error of the media drive train is mapped and stored as a look-up table in a non-volatile memory, enabling the printer control electronics to compensate for predicted error by referencing an error map. The media drive system delivers media advance accuracy to about 0.0001 inch for print media up to about xc2xc-inch thick. This advance accuracy matches or exceeds that of prior art print engines, at a similar cost-of-manufacture and accommodates the much faster print speeds and higher ink lay-down rates required by the preferred high-resolution inkjet print cartridge.
The printer control system preferably employs two electronics subassemblies: the first, disposed in an off-carriage electronics bay, runs the operating system software and performs all I/O, housekeeping and print engine control functions. The second, disposed on-carriage the carriage assembly, performs data management and control operations related to transmitting image data to the print heads. The off-carriage printer control electronics connect to the media thickness sensor, a low-cost high resolution apparatus that includes a potentiometer and moment arm that sense the presence of media and can measure its thickness to a precision of about 0.0001-inch. The printer control system is responsive to analog signals generated from the media thickness sensor, as well as periodicity error data stored in its memory. The printer control system references this data to regulate the media handling system such that a print medium is accurately advanced under a constant tension for each type of media used. Media thickness data is also used to set the head-height position of the carriage penholder. The off-carriage printer control electronics stores information about the media type, roll length and media thickness in non-volatile, on-carriage memory and monitors the print medium remaining on the supply roll. It uses the stored data to calculate and record the media type and amount of media remaining on an unused portion of the roll, as well as to notify the operator of an inadequate supply for a requested print job. It also automatically recalls the media advance and head-height settings for future use, such as when any similar type of media is loaded into the printer or for reference by an operator. The printer control electronics also performs a series of checks, to detect any deficiencies in printed output, and as series of calibrations, to compensate for deficiencies that might evidence as irregularities or artifacts in the printed image. A series of different test imagesxe2x80x94such as registration targets and color chartsxe2x80x94are printed on a pre-selected print medium.
The improved image sensor assembly on-board the scanning carriage captures and transmits information about the test images to the printer control electronics. This information is used to perform a series of calibrations to compensate for various conditions, including misfiring and failed jets, print-head misalignment, inconsistency in the interval spacing of encoder strip indicia, variation in dot placement accuracy for pixel locations serviced by different jetting nozzles, media advance inaccuracy, and changes in color consistency. Each of these checks and compensatory calibrations can be performed on-demand by an operator or at a scheduled interval chosen by an operator. In addition, the image sensor is capable of performing some tests and checks during printing operation, providing means for the printer control electronics to continually monitor print quality and automatically compensate for deficiencies as quickly as they are detected. This capability, in turn, provides a welcome benefit of greater latitude in performing unattended printing, since operators may have greater confidence in the quality of printed output therefrom. The image sensor is equipped with a fast, chromatically tuned (c.f. sensitive to the visible light spectra) photodiode that performs color-metric measurement of test color charts. The printer control electronics uses the color measurement data to compensate for changes in color consistency that may occur, for example, as print heads age over time and the jetting nozzles therein become worn or fatigued. These conditions cause variations in the volume of ink that is emitted from a jetting nozzle and/or the response time of a nozzle to a fire pulse that evidence as changes in color hue. Other causes of inconsistent color might relate to differences between print media of the same type caused by small variations in coating chemistry and porosity, changes in relative humidity, and so forth. The use of a fast photodiode enables the image sensor to automatically perform color-metric quality tests and undertake compensatory action for color variations from a norm or due to inconsistency. Additionally, the image sensor can be used to characterize the interaction of a particular set of process ink colors with a particular media, since the photodiode is an accurate measurement tool of chromatic constituents. This capability, in turn, allows rapid and efficiency characterization of new types of media installed in the printer, without requiring recourse to an external color-metric device or apparatus.
In summation, the improved inkjet printer taught herein incorporates a number of novel design features that augment its usefulness and operating simplicity resulting in significant advantages overall. Several of the key benefits of the present invention include eliminating critical adjustments in the field, performing automatic monitoring ofxe2x80x94and compensation forxe2x80x94printing deficiencies, efficient replacement of marking system components, and rapid changeover between different sets of inks or ink types. Each of these benefits reduces the level of operator intervention required to make efficient use of the inkjet print instant. The present invention achieves these goals so that advanced, large-format digital color printers may be reliably and simply fabricated, operated and servicedxe2x80x94and thereby produced in high volumes at reduced cost of ownership and making such machines less expensive overall.
Other features of the invention are described below.